Can I use splines and bones together in the same rig?
There is nothing stopping you from doing both.  This situation comes up from time to time when rigging polygonal hair to a character: the hair usually has spline control, and the body is made of bones.  Just make sure your parenting hierarchy and weight paint makes sense.  For simplicity, you may want to make the tail and body two discreet objects with their own deform modifier. 

Also, there is the spline IK modifier for bones, in which a spline controls a series of bones.  So it's kind of mixing both.